Crystallinelike Ordering of Confined Liquids at the Moving Contact Line

The friction between a liquid swollen soft elastomer and a solid surface depends on the state of a confined liquid. To measure the physical state of the confined liquid, an interface-sensitive sum frequency generation spectroscopy technique was used to probe the contact region. We find that during sliding (friction) and pull-off (adhesion) experiments of pentadecane-swollen poly(dimethyl siloxane) lenses submerged in linear alkane (pentadecane) on a sapphire substrate, crystallinelike ordering of the liquid occurs only at the contact line, where we anticipate the highest shear. This crystallinelike structure of pentadecane molecules is transient and shows Arrhenius temperature dependence with unusually long relaxation times (hundreds of seconds) and an activation energy (50  kJ/mole), which is twice that of the bulk pentadecane liquid, at temperatures that are 14-70 °C higher than the bulk melting temperature (T_{m}=9 °C). This unusual long-lived crystallinelike ordering may explain why these systems show higher friction coefficients (boundary lubrication) compared to values predicted using bulk viscosity of pentadecane (hydrodynamic lubrication).

Melting of linear alkanes between swollen elastomers and solid substrates

We have measured the melting and freezing behavior of linear alkanes confined between cross-linked poly(dimethylsiloxane) (PDMS) elastomers and solid sapphire substrates. Small molecules are often used as lubricants to reduce friction or as plasticizers, but very little is directly known about the migration or changes in physical properties of these small molecules at interfaces, particularly the changes in transition temperatures upon confinement. Our previous studies highlighted striking differences between the crystal structure of confined and unconfined pentadecane crystals in contact with sapphire substrates. Here, we have used surface-sensitive infrared-visible sum-frequency-generation spectroscopy (SFG) to study the melting temperatures (Tm) of alkanes in nanometer thick interfacial regions between swollen PDMS elastomers in contact with sapphire substrate. We find that confined alkanes show depression in Tm compared to the melting temperature of unconfined bulk alkanes. The depression in Tm is a function of chain length, and these differences were smallest for shorter alkanes and largest for 19 unit long alkanes. In comparison, the DSC results for swollen PDMS elastomer show a broad distribution of melting points corresponding to different sizes of crystals formed within the network. The Tm for confined alkanes has been modeled using the combination of Flory-Rehner and Gibbs-Thomson models, and the depression in Tm is related to the thickness of the confined alkanes. These findings have important implications in understanding friction and adhesion of soft elastomeric materials and also the effects of confinement between two solid materials.

Confinement-induced ordering of alkanes between an elastomer and a solid surface

We have studied the molecular structure of liquid alkanes confined between a flexible elastomeric poly(dimethyl siloxane) lens and a rigid sapphire substrate using surface-sensitive infrared-visible sum frequency generation spectroscopy. The reduction in the gauche defects suggests ordering of liquid alkanes under confinement. The cooling of confined liquid below the freezing temperature leads to crystallization with alkane molecules lying on the substrate with the symmetry axis parallel to the surface normal. This structure is very different from the bulk alkane crystals next to sapphire or air interfaces.